The present invention relates to a technique which detects an object on the basis of information from a transmitted wave and a received wave.
When an object is detected using a radar device, a transmitted signal obtained from a transmitted wave and a received signal obtained from a received wave are mixed, and the relative distance and relative speed of the object are detected from a peak signal detected by the difference in frequency of the transmitted signal and the received signal.
In detecting the relative distance and relative speed of the object from the peak signal, a beat signal which is generated by mixing the transmitted signal and the received signal is A/D converted by an A/D converter (Analog to Digital Converter) and input to a signal processing device, such as a microcomputer. Then, the signal processing device carries out FFT (Fast Fourier Transform) processing for the beat signal to detect a frequency spectrum.
In general, the frequency spectrum of the object has a relatively larger power level than the frequency spectrum of noise, or the like, thus a frequency spectrum which exceeds a threshold value at a predetermined power level is detected as the peak signal. Then, the relative distance and relative speed of the object are detected on the basis of the detected peak signal.
In detecting the relative distance and relative speed of the object, when the level of the transmitted signal is set to be constant, the power level of the received signal has variations depending on the reflection sectional area of the object, the distance from the radar device to the object, and the relative speed of the vehicle with the radar device and the object. In particular, the power level of a received signal of an object having a short relative distance increases compared to the power level of a received signal of an object having a long relative distance. If the power level of the received signal increases and then exceeds the saturation level of a receiving circuit including a mixer or an A/D converter, the beat signal is sampled as a substantially square wave.
Then, the FFT processing is carried out for the beat signal which is sampled as a substantially rectangular wave, such that a corresponding frequency spectrum is detected.
The frequency spectrum which is generated from the beat signal as a substantially square wave includes a frequency spectrum related to an existent object and a frequency spectrum of an integer multiple related to the object. Then, the power level of the frequency spectrum generated from the beat signal as a substantially square wave is greater than the level of another frequency spectrum in correspondence with the saturation level of the receiving circuit including the A/D converter, and thus exceeds the threshold value for detection of the peak signal. As a result, the relative distance and relative speed of a non-existent object may be erroneously detected, which may cause a problem for vehicle control.
When the relative distance from the object is short, part of the transmitted wave is multiple-reflected by the object, and a beat signal detected by single reflection from the object and a beat signal detected by multiple reflection from the object are generated. If a frequency spectrum corresponding to the beat signal detected by multiple reflection from the object exceed the threshold value for detection of the peak signal, a peak signal which is non-existent as an object is detected. As a result, the relative distance and relative speed of the non-existent object may be erroneously detected, which may cause a problem for vehicle control.
With regard to the above-described problem, in the related art, a method is suggested which decreases the level of the transmitted signal in advance to decrease the level of the received signal.
Further, a method is suggested which uses a receiving circuit having a large saturation level to prevent an integer-multiplied frequency spectrum from being detected as a peak signal (for example, JP-A-11-133144).
However, the threshold value for detection of power of the frequency spectrum of the object having a short relative distance from the vehicle is set to be greater than the threshold value for detection of power of the frequency spectrum of an integer multiple of the peak signal of the object having a short relative distance. This is to prevent noise from being erroneously detected as the peak signal of the object since a frequency spectrum generated by the effect of noise due to frequency modulation of the transmitted wave or the received wave or noise from hardware equipment is generated a lot in a low frequency band.
Accordingly, if the level of the received signal decreases, the frequency spectrum of the object having a short relative distance has a large threshold value for detection of power of the frequency spectrum, thus the frequency spectrum falls below the threshold value and is not detected as a peak signal. The frequency spectrum of the integer multiple of the peak signal of the object having a short relative distance has a small threshold value, thus the frequency spectrum exceeds the threshold value and is detected as a peak signal. For this reason, during pairing processing in which peak signals in predetermined periods at the time of transmission/reception are paired so as to detect an object, erroneous pairing may be carried out for a peak signal related to a non-existent object and a peak signal related to an existent object. As a result, erroneous vehicle control may be performed. In addition, while a non-existent object is detected, an object having a short relative distance from an existent vehicle may not be detected. As a result, during continuous determination processing in which it is determined whether information related to the same object is continuously detected or not, even when an object is actually existent, object detection is impossible, thus an object may not be continuously detected and not subject to vehicle control. Further, the use of the receiving circuit having a large saturation level described in JP-A-11-133144 leads to an undesirable increase in cost of the radar device.